1. Field of the Invention
The present invention generally relates to living tissue stimulators and biotelemetry devices and, more particularly, to a living tissue interacting device of the type that includes a power source, which powers circuitry and a memory in which parameters for controlling the operation of the circuitry are stored.
2. Description of the Prior Art
Various devices are presently in existence which are implantable in a human patient to stimulate and/or detect signals from body tissues. Among the best known of these devices is the cardiac pacemaker, which provides stimulating pulses to a patient's heart and detects endocardial potentials, via one or more electrodes, which are connected to the pacemaker through electrode leads. Some of the existing implantable cardiac pacemakers include a power source in the form of a battery, which is rechargeable by means of recharging power, transmitted to the implanted pacemaker from an external power source.
In recent years extensive research has been directed to develop stimulators and signal detectors for other than the heart, in order to relieve patients of the crippling effects of various physiological disorders. For example, stimulators and signal detectors have been proposed to stimulate and detect signals from the brain, the spine, muscles, glands and organs or any other tissue. The detected signals and the stimulating pulses from these devices are intended to help patients suffering from various disorders, e.g., cerebral palsy, spasticity, rigidity, epilepsy and other disorders, which are due to either improper, or the absence of, natural stimulating pulses. The detected signals are either used to control the stimulation and/or to inform the doctor of the status of the tissue. Also, it has been appreciated that pain, such as phantom limb pain, resulting from a severed limb, may be alleviated by applying stimulating pulses to the nerves proximal to the damaged area. A workshop was held at the National Institute of Health, Bethesda, Md. on Apr. 27-28, 1972, and a report of the workshop entitled "Functional Neuromuscular Stimulation" was published in 1972 by the National Academy of Sciences, Washington, D.C.
Different disorders require different stimulations. For example, the various properties or characteristics of the stimulating pulses, such as pulse amplitude, pulse frequency, pulse width, and other pulse properties have to be different for different disorders. Also, they may differ from patient to patient. Furthermore, even for the same patient, the pulses' properties may have to be varied, depending on the patient's condition at any given time. Clearly, it would be prohibitively expensive to fabricate a customized stimulator and signal detector for each patient. Furthermore, even if tailor-made for a specific patient, the stimulator would have to be capable of varying the pulses' properties in order to vary the stimulation and to vary the characteristics of the signal detector to suit the patient's changing conditions.
The only practical solution is to provide an implantable stimulator and signal detection system, hereinafter referred to as a human tissue stimulator or HTS, with a programmable memory in which multibit words, hereinafter referred to as parameters, are storeable. At least some of these parameters may be used to control different properties of the stimulating pulses and the signal detector. By varying one or more of the parameters in the memory, different properties of the stimulating pulses and the signal detector may be changed. Any one of the parameters in the memory may be changed by transmitting appropriate signals from a source, external to the body, which after being received in the HTS may be decoded and, when found to be proper, may be supplied to the memory to replace a corresponding, previously stored, parameter in the memory. Since the manner of changing parameters in a memory does not form part of this invention, it will not be described in any further detail.
As is appreciated by those familiar with the art, all memories can be classified as either volatile or non-volatile. A non-volatile memory is one in which the stored parameters are not affected by changes of the voltage which is supplied to the memory from a power source, e.g., a battery, which powers the memory. On the other hand, a volatile memory is one which can be relied upon as operating properly only when the voltage which is supplied thereto does not fall below an appropriate level. If however, the voltage from the power source falls below such a level, any parameter stored in the memory can no longer be relied upon since it may have changed, i.e., destroyed, when the voltage to the memory was insufficient for its appropriate operation.
In an HTS with a memory which stores parameters which in turn control the operation of the HTS, e.g., the properties of the stimulating signals, since the patient's safety in most instances depends on the properness of the stimulating pulses, it is paramount to insure that any or all of the stored parameters do not change in an arbitrary manner due to memory power failure. Such changes may endanger the patient's life. One way which this may be accomplished is to store the parameters in a non-volatile memory. If the parameters are to be stored in a volatile memory, some means must be provided to either protect the memory power supply and/or, if this cannot be done, to reset the memory to prevent dangerous stimulating regimes. Such means are hereinafter referred to as the memory protect means or circuit.